OK, it's not 100% accurate and it will not give any HP numbers but you can test mods to see if you have any gain. Here's the method:

- find a nice straight piece of road with good visibility and at least a 55 mph speed limit. It works best if the road is uphill.
- find a good roadside marker as your starting point.
- when traffic clears, roll to the starting point in 3rd gear at 2000rpm. (setting the cruise control for 2000rpm will give you a good consistent "launch" at 2000rpm)
- hit the stopwatch and floor the throttle at the same time.
- If you have enough road and a 65-70mph speed limit, stop the time when you reach redline. The best road I have near my house is a 55mph zone so I am stopping the watch at 6000rpm to avoid pissing off people that live on that road or getting a ticket.

There you have it. The only factor that you really don't control is air condition.

I tried the same method with 2nd gear 2000rpm to redline but it was hard to get consistent times (varied from 6.1 to 6.5 sec). I found I could get much more consistent times using 3rd from 2000-6000rpm (very repeatable 12.2). Of course your times will be different on a different road. Again, it helps to find an uphill road to lengthen the time, giving more margin for error.